Effect of A2O-MBBR + CWs Combined Technology for Treating Rural Domestic Wastewater
In recent years, the state has been deeply promoting the rural revitalization development strategy, focusing on improving the living environment, and placing higher demands on rural domestic wastewater treatment. Currently, the main processes for rural domestic wastewater treatment include biological methods, ecological methods, and combined processes, most of which originate from urban wastewater treatment. However, rural areas are characterized by scattered populations, leading to numerous problems such as high dispersion of wastewater, difficulty in collection, small treatment scales, low resource utilization rates, and insufficient treatment facilities. Furthermore, significant differences exist in wastewater quality and quantity, geographical location, climate, and economic levels across regions, making it difficult to standardize treatment technologies; simple adoption of urban wastewater treatment technologies is not feasible. The infrastructure for wastewater collection, such as sewer networks, is often inadequate in rural areas. Wastewater collection is easily affected by combined sewer overflows and groundwater infiltration, resulting in low organic concentration in the wastewater and increased difficulty for biological nitrogen removal. The large fluctuations in wastewater quality and quantity in rural areas make it difficult to maintain stable biomass concentration in treatment facilities. Moreover, low winter temperatures limit biological treatment capacity, leading to low efficiency and unstable effluent quality prone to exceeding standards in traditional activated sludge processes. Therefore, there is an urgent need to develop wastewater treatment technologies suitable for local conditions, with strong resistance to shock loads, stable long-term operation, low energy consumption, and high treatment efficiency.
Rural areas in China tend to prefer low-cost, easy-to-manage domestic wastewater treatment technologies, with biological + ecological combined processes being a major research direction. Currently, widely used integrated packaged wastewater treatment equipment in rural areas mainly employs processes such as Anaerobic-Anoxic-Oxic (A2O) and Moving Bed Biofilm Reactor (MBBR). Studies show that the MBBR process relies more on facility design than on precise operational control, requiring no professional technical personnel for regulation, making it convenient for operation and maintenance. This is more suitable for the practical needs of rural domestic wastewater treatment where technical personnel are scarce. Its advantages include high biomass concentration, strong resistance to shock loads, high treatment efficiency, and small footprint. Research by Luo Jiawen et al. indicates that adding MBBR media to the A2O process can significantly improve its wastewater treatment capacity. Zhou Zhengbing et al., in an actual rural domestic wastewater project, designed a two-stage anaerobic/anoxic-biological aerated filter combined process, achieving stable effluent quality meeting the Grade A standard of GB 18918-2002 "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants". Additionally, Constructed Wetlands (CWs) are often used for rural domestic wastewater treatment. For example, Zhang Yang et al. used biochar as a filler to modify a constructed wetland, finding removal rates for TN, TP, and COD could reach 99.41%, 91.40%, and 85.09%, respectively. Previous research by our group also showed that sludge biochar filler could enhance the nitrogen and phosphorus removal performance of constructed wetlands, improving the overall system's treatment efficiency and effectiveness, and making the system more resistant to shock loads. Building on the above research, to explore a combined technology suitable for rural domestic wastewater treatment and address challenges such as difficulty in maintaining stable biomass concentration, weak resistance to shock loads, and effluent quality prone to fluctuations and exceeding standards in rural wastewater treatment facilities, the author placed an A2O-MBBR process upfront, filling it with suspended biofilm carriers to create an integrated fixed-film activated sludge (IFAS) environment, increasing system sludge concentration and enhancing treatment efficiency. Considering the ecological utilization of available idle land like ponds and depressions in rural areas, and combining constructed wetlands as a polishing treatment process, methods such as using sludge biochar filler, recirculating nitrified liquid, and planting submerged plants were employed to enhance the operational stability of the composite wetland. Thus, an A2O-MBBR + CWs combined process was constructed.
In this study, using raw wastewater from a village wastewater treatment plant in Hefei as the treatment object, a pilot-scale experimental setup of the A2O-MBBR + CWs combined process was constructed. The influence of seasonal water temperature changes on its treatment performance was investigated. Pollutants indicators in the influent and effluent were monitored during operation to explore removal efficiencies and operational stability. Simultaneously, the economic feasibility of the process was analyzed. The aim is to provide data reference and basis for the application of A2O + constructed wetland combined technology in rural domestic wastewater treatment projects in China, and to offer references for promoting domestic wastewater treatment and building beautiful, ecologically livable villages in rural areas.
1. Experimental Setup and Research Methods
1.1 Combined Process Flow
The A2O-MBBR + CWs combined process experiment adopted a series operation of an A2O unit, a carbon-based subsurface flow wetland, and an ecological pond. The A2O unit consisted of a baffled anaerobic-anoxic contact tank and an aerobic membrane tank (MBBR). Both the baffled anaerobic tank and the aeration zone of the aerobic MBBR tank were filled with suspended biofilm carrier media to provide attachment surfaces for microorganisms to form biofilms. The activated sludge and biofilm in the tanks coexisted, forming an IFAS system, which could stably maintain system biomass. The baffled anoxic tank enhanced the denitrification process through nitrified liquid recirculation. The aerobic MBBR tank had an aeration system at the bottom to enhance its nitrification performance. A Poly Aluminum Chloride (PAC) dosing port was set inside the tank for supplemental chemical phosphorus removal, enabling efficient phosphorus removal. The CWs unit included a carbon-based subsurface flow wetland and a submerged plant ecological pond. The carbon-based subsurface flow constructed wetland adopted a three-stage filler filtration system. Aeration discs were installed at the bottom of the filler zone for backwashing the media to mitigate clogging. The submerged plant ecological pond had a limestone substrate layer at the bottom and was planted with cold-tolerant submerged plants Vallisneria natans and Potamogeton crispus. The setup was placed outdoors. A thermometer was installed in the ecological pond to monitor seasonal water temperature changes. The detailed process flow of the A2O-MBBR + CWs combined process is shown in Figure 1.
1.2 Setup Design and Operational Parameters
The experimental setup was constructed using 10 mm thick polypropylene plates. The baffled anaerobic tank was filled with square biofilm carrier media and contained baffle plates. The mixed liquor recirculation ratio for the baffled anoxic tank was 50%~150%, and it also contained baffle plates. The aerobic MBBR tank was divided by a baffle into an aerobic aeration zone and a sedimentation zone. The aeration zone was filled with MBBR suspended carrier media with an air-to-water ratio of 6:1~10:1. The sedimentation zone had a PAC dosing port and inclined plates for sedimentation aid. The carbon-based subsurface flow wetland: the primary filler zone was filled with limestone (~5 cm diameter), the secondary filler zone with zeolite (~3 cm diameter), and the tertiary filler zone with sludge biochar filler (~0.5~1.0 cm diameter). The filler height for each zone was 75 cm. A gap zone about 4 cm wide was set between the primary and secondary filler zones for functions such as adding external carbon sources, observation, and maintenance/emptying (no carbon source was added during this experiment). The submerged plant ecological pond was filled with limestone filler (~3 cm diameter) at a height of 20 cm. Submerged plants were planted at a row spacing of 10 cm and plant spacing of 10 cm. The experiment used raw wastewater from a village wastewater treatment plant in Hefei as influent. The experimental period was from May 25, 2022, to January 17, 2023, totaling 239 days. Submerged plants were harvested once on December 2nd, with a frequency of approximately once every 6 months. The designed wastewater treatment capacity was 50~210 L/d. Detailed design parameters of the setup are shown in Table 1.
1.3 Experimental Methods
1.3.1 Experimental Design
1.3.1.1 Optimal Wastewater Treatment Capacity Test
After successful trial operation of the experimental setup (stable effluent quality), the optimal wastewater treatment capacity test was conducted from May 25, 2022, to June 30, 2022. Under conditions of maintaining an aerobic tank air-to-water ratio of 6:1, nitrified liquid recirculation ratio of 100%, and PAC (Al2O3 content 28%) usage of about 3.7 g/d, the wastewater treatment capacity of the setup was gradually increased (50, 60, 70, 80, 100, 120, 150, 180, 210 L/d). Changes in effluent quality were monitored to explore the optimal wastewater treatment capacity of the setup. During this period, water temperature varied between 24.5~27.1°C. To ensure stable effluent compliance in winter, the effluent standard adopted was the Grade A standard of GB 18918-2002 "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants".
1.3.1.2 Combined Process Overall Treatment Performance Test
The test period was from July 1, 2022, to January 17, 2023. The optimal wastewater treatment capacity was set at 120 L/d. The aerobic tank air-to-water ratio was 6:1~10:1, and the mixed liquor recirculation ratio was 50%~150%. Influent and effluent water quality indicators (TN, TP, NO3--N, NH4+-N, and COD) from each process unit were monitored. Water temperature changes during the test period (influenced by seasonal climate) were recorded. The treatment performance of the A2O-MBBR + CWs combined process for rural domestic wastewater was analyzed, and the influence of seasonal water temperature changes on the combined process's performance was investigated.
1.3.2 Sampling
During the test period, samples were taken irregularly (approximately 1~2 times per week) for water quality testing. Samples were collected from the setup influent, baffled anaerobic-anoxic tank effluent, aerobic MBBR tank effluent, carbon-based subsurface flow wetland effluent, and submerged plant ecological pond effluent. Influent samples were taken from the setup's inlet pipe, and effluent samples from each unit's outlet. Water quality indicator testing was completed on the same day of sampling. Tested indicators included TN, TP, NO3--N, NH4+-N, and COD. Each time samples were taken, the water temperature reading from the thermometer in the ecological pond was recorded (varying between 0~32°C). The water temperature in the ecological pond changed naturally with seasonal temperature differences. The designed effluent standard for the experimental setup followed the Grade A standard of DB 34/3527-2019 "Discharge Standard of Water Pollutants for Rural Domestic Wastewater Treatment Facilities". The designed influent concentrations and effluent standards are detailed in Table 2.
1.3.3 Water Quality Analysis Methods
TN concentration in water samples was determined using HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion UV spectrophotometric method". NO3--N concentration was determined using HJ/T 346-2007 "Water quality - Determination of nitrate nitrogen - Ultraviolet spectrophotometry (Trial)". NH4+-N concentration was determined using HJ 535-2009 "Water quality - Determination of ammonia nitrogen - Nessler's reagent spectrophotometry". COD was determined using HJ 828-2017 "Water quality - Determination of chemical oxygen demand - Dichromate method". TP concentration was determined using GB 11893-1989 "Water quality - Determination of total phosphorus - Ammonium molybdate spectrophotometric method".
2. Results and Discussion
2.1 Influence of Wastewater Treatment Capacity on Combined Process Performance
As shown in Figure 2 (a)(b), as the daily wastewater treatment capacity gradually increased from 50 L/d to 210 L/d, the removal efficiencies of TN and NH4+-N by each unit of the combined process showed a decreasing trend. TN removal rate decreased from 91.55% (50 L/d) to 52.17% (210 L/d), and NH4+-N removal rate decreased from 97.47% (70 L/d) to 80.68% (210 L/d). This is because the increase in daily wastewater treatment capacity reduces hydraulic retention time, shortening the time available for microorganisms to degrade pollutants, resulting in poorer treatment performance. Among them, the A2O unit contributed the most to TN and NH4+-N removal. The average influent TN concentration for this unit was 38.68 mg/L, effluent was 16.87 mg/L, with a removal rate of 56.29%. The average influent NH4+-N concentration was 36.29 mg/L, effluent was 5.50 mg/L, with a removal rate of 84.85%. For the carbon-based subsurface flow wetland, the average influent TN concentration was 16.87 mg/L, effluent was 11.96 mg/L, with a removal rate of 29.10%. For the submerged plant ecological pond, the average influent TN concentration was 11.96 mg/L, effluent was 9.47 mg/L, with a removal rate of 20.82%. The nitrogen removal performance of the carbon-based subsurface flow wetland was better than that of the ecological pond because the anaerobic-anoxic environment of the subsurface flow wetland is more suitable for denitrification. However, the NH4+-N removal performance of the ecological pond was better than that of the subsurface flow wetland. The average influent NH4+-N concentration for the carbon-based subsurface flow wetland was 5.50 mg/L, effluent was 4.04 mg/L, with a removal rate of only 26.53%. For the ecological pond, the average influent NH4+-N concentration was 4.04 mg/L, effluent was 2.38 mg/L, with a removal rate of 41.07%. This is because the aerobic environment of the ecological pond is more suitable for nitrification, converting more NH4+-N into NO3--N, resulting in a higher NH4+-N removal rate. When the wastewater treatment capacity reached 150 L/d, the effluent TN concentration was 15.11 mg/L, exceeding the Grade A standard of GB 18918-2002. Therefore, to ensure stable TN compliance, the maximum wastewater treatment capacity was 120 L/d. When the wastewater treatment capacity reached 210 L/d, the effluent NH4+-N concentration was 7.07 mg/L, exceeding the Grade A standard of GB 18918-2002. Therefore, the maximum wastewater treatment capacity for NH4+-N compliance was 180 L/d.
As shown in Figure 2 (c), the average influent COD was below 100 mg/L, indicating low organic content. The increase in wastewater treatment capacity did not significantly affect COD removal, with COD removal rates between 75%~90%. As the wastewater treatment capacity increased from 50 L/d to 210 L/d, the average effluent COD was 19.16 mg/L, with a maximum effluent COD of 26.07 mg/L, still far below the 50 mg/L standard of GB 18918-2002 Grade A. The A2O unit contributed the most to COD removal because the aeration device in the aerobic MBBR tank created an aerobic environment, enhancing the biochemical capacity of aerobic microorganisms and strengthening COD removal. Additionally, the recirculation of nitrified liquid in the A2O unit allowed the baffled anoxic tank to further utilize organic matter in the wastewater as a carbon source, removing part of the COD while enhancing denitrification. The carbon-based subsurface flow wetland contributed the second most to COD removal. Its anaerobic-anoxic environment is conducive to using organic matter in the wastewater as a carbon source, degrading part of the organics while enhancing denitrification, which is also why it had better TN removal. Furthermore, the substrate layer of the subsurface flow wetland can adsorb some organic matter. The ecological pond had limited effect on COD degradation. The average influent COD for the ecological pond was 22.21 mg/L, and most easily biodegradable organics had already been degraded, leaving organics that are more difficult to degrade.
As shown in Figure 2 (d), as the wastewater treatment capacity increased, the effluent TP concentration remained stable. The increase in wastewater treatment capacity did not significantly affect TP removal. The average influent TP concentration was 3.7 mg/L, and the average effluent concentration was 0.18 mg/L, with an average removal rate of 95.14%, indicating good TP removal. TP was mainly removed in the A2O unit. The influent TP concentration for the A2O unit was 3.7 mg/L, and the effluent was only 0.29 mg/L, better than the 0.5 mg/L standard of GB 18918-2002 Grade A. This is because the A2O unit not only had biological phosphorus removal by phosphorus-accumulating organisms (PAOs) but also supplemented with chemical phosphorus removal by dosing 3.7 g/d of PAC. The combination of biological and chemical phosphorus removal resulted in over 90% of phosphorus being removed in the A2O unit. The subsurface flow wetland and ecological pond mainly relied on mechanisms such as substrate adsorption, sedimentation, plant uptake, and microbial degradation for phosphorus removal. Moreover, the TP concentration entering the wetland was already as low as 0.29 mg/L, making further removal more difficult. These combined reasons led to the general TP removal performance of the wetland and ecological pond.
Therefore, to ensure stable compliance of all effluent indicators with the GB 18918-2002 Grade A standard, the optimal wastewater treatment capacity for this process was determined to be 120 L/d.
2.2 Pollutant Removal Performance of the Combined Process
2.2.1 COD Removal Performance
As shown in Figure 3, during the overall treatment performance test period (July 1, 2022, to January 17, 2023, wastewater treatment capacity 120 L/d), water temperature showed a fluctuating downward trend, decreasing from 32°C to 0°C. The COD removal rate fluctuated, and the decrease in water temperature had no obvious impact on COD removal. Combined with Figure 4, the COD removal rate varied between 66.16%~82.51%, primarily influenced by influent COD concentration. Studies show that under anaerobic/anoxic conditions, COD removal mainly relies on microbial action. The A2O-MBBR+CWs process alternates between anaerobic-anoxic-oxic-anoxic-oxic conditions, enhancing COD removal. During operation, as water temperature decreased, although influent COD ranged from 80~136 mg/L, effluent COD remained stable below 50 mg/L, meeting the Grade A standard of DB 34/3527-2019, indicating good organic degradation. The A2O section contributed the most to COD removal. The baffled anaerobic-anoxic contact tank had an average COD removal rate of 43.38%, accounting for 65.43% of the total COD removal. The aerobic MBBR tank had an average removal rate of 14.69%, accounting for 19.87% of the total. The A2O section contributed over 85% to COD removal, benefiting from the large specific surface area of the media in the baffled anaerobic tank and aerobic MBBR tank, high sludge concentration, and the formation of a food chain from bacteria → protozoa → metazoa, effectively degrading organic matter in water. The high biodiversity of the IFAS system ensured good organic removal even with temperature changes. Additionally, part of the soluble organic matter in the wastewater in the baffled anaerobic-anoxic contact tank would be used as a carbon source by denitrifying bacteria. Meanwhile, recirculated mixed liquor increased NO3--N concentration in the baffled anoxic tank, promoting the utilization of carbon sources by denitrifying bacteria to convert NO3--N/NO2--N into nitrogen gas. The high COD removal rate in the baffled anaerobic-anoxic contact tank further verifies that this process can efficiently utilize organic matter in wastewater as a denitrification carbon source. The carbon-based subsurface flow wetland had an average COD removal rate of 7.18%, accounting for 9.18% of total COD removal. The anaerobic/anoxic environment of the subsurface flow wetland is conducive to microorganisms using organic matter as a carbon source, achieving COD removal while enhancing denitrification. Related research also indicates that biochar filler can adsorb organic matter through electrostatic attraction and intermolecular hydrogen bonding. Therefore, the sludge biochar filler in the subsurface flow wetland would also adsorb some organic matter. The submerged plant ecological pond had an average COD removal rate of only 3.68% because the COD entering the pond was already low at 30.59 mg/L on average, and mostly consisted of refractory organics, removed mainly by adsorption and plant uptake, with limited effect.
2.2.2 Nitrogen Removal Performance
As shown in Figure 3, as water temperature gradually decreased from 32°C to 12°C, TN and NH4+-N removal rates fluctuated. The average TN removal rate reached 75.61%, and the average NH4+-N removal rate reached 95.70%. When water temperature dropped below 12°C, TN and NH4+-N removal rates showed a rapid decreasing trend, but average removal rates still reached 58.56% and 80.40%, respectively. This is because seasonal water temperature decrease inhibited microbial activity, weakening denitrification performance. According to the statistical results of influent and effluent pollutant concentrations during the combined process operation period (July 1, 2022, to January 17, 2023) shown in Table 3, the average influent TN and NH4+-N concentrations were 36.56 mg/L and 32.47 mg/L, respectively. NH4+-N accounted for 88.81% of TN. Influent NO3--N (0.01 mg/L) was almost negligible. Average effluent TN and NH4+-N concentrations were 11.69 mg/L and 3.5 mg/L, respectively, both meeting the Grade A standard of DB 34/3527-2019. The average effluent NO3--N concentration was 6.03 mg/L, indicating good nitrification capacity of this process, converting NH4+-N to NO3--N. However, the accumulation of NO3--N in the effluent suggests there is still room for further denitrification. As shown in Figure 5 (a), TN removal was highest in the A2O section. The baffled anaerobic-anoxic contact tank had an average TN removal rate of 44.25%, and the aerobic MBBR tank had an average TN removal rate of 9.55%. This is the result of the combined action of nitrifying bacteria in the aerobic zone and denitrifying bacteria in the anoxic zone. The carbon-based constructed wetland had an average TN removal rate of 11.07%, because its ability to release carbon sources and its anaerobic/anoxic environment are conducive to denitrification, maintaining a certain nitrogen removal capacity. The submerged plant ecological pond had an average TN removal rate of only 3.54%, with general removal performance, because its aerobic environment is not conducive to denitrification. As shown in Figure 5 (b), NH4+-N removal was primarily completed in the A2O section. The baffled anaerobic-anoxic contact tank had an NH4+-N removal rate of 59.46%, and the aerobic MBBR tank had an NH4+-N removal rate of 24.24%. The A2O section accounted for 93.57% of the total NH4+-N removal. The high NH4+-N removal in the A2O section is due to continuous aeration in the aerobic MBBR tank, allowing nitrifying bacteria to fully utilize DO to convert NH4+-N to NO3--N. This is then recirculated to the anoxic tank, where denitrifying bacteria convert NO3--N to N2 for removal. During the test period, the average TN removal rate was 68.40%, and the average NH4+-N removal rate was 89.45%, indicating good nitrogen removal performance.
As shown in Figure 3, as water temperature decreased from 32°C to 0°C, the TN removal rate decreased from a maximum of 79.19% to 51.38%. Combined with Figure 5 (a), when water temperature was >20°C, the average TN removal rate exceeded 75%, with an average effluent concentration of 8.41 mg/L, because microbial activity is higher in the 20~32°C range, leading to better denitrification, consistent with research by Zhang Na et al. When water temperature decreased from 20°C to 5°C, the average TN removal rate decreased to 65.44%, and the average effluent concentration increased to 12.70 mg/L. When water temperature was 0~5°C, the average TN removal rate decreased to 52.75%, and the average effluent concentration increased to 17.62 mg/L, indicating a certain impact on TN removal. Studies show that as water temperature decreases, microbial activity is inhibited. When water temperature <5.6°C, microorganisms are basically dormant, and population numbers sharply decrease, limiting pollutant degradation. When water temperature <4°C, microorganisms begin to die. However, in this process, even when water temperature dropped to 0°C, the TN removal rate still reached 51.52%, and effluent always met the Grade A standard of DB 34/3527-2019. This is because the IFAS system in the A2O section maintained high biomass concentration. During the test period, MLSS concentration in the baffled anaerobic-anoxic contact tank and aerobic MBBR tank reached 6,000~8,000 mg/L. Additionally, recirculation of nitrified liquid further enhanced denitrification. Furthermore, wastewater passed sequentially through the limestone, zeolite, and sludge biochar filler zones of the subsurface flow wetland, where anaerobic and aerobic reactions occurred simultaneously. Various organics adsorbed on filler surfaces and the slow-release of carbon sources from biochar filler promoted denitrification, further enhancing nitrogen removal. Research indicates that biochar can increase the abundance and diversity of denitrifying microorganisms in wetlands. Also, due to its structure, subsurface flow wetlands have some thermal insulation effect, helping maintain internal microbial activity. Under the influence of multiple factors, the combined process exhibited strong resistance to low-temperature shock, maintaining over 50% TN removal even at 0°C. In summary, when water temperature is >5°C, TN removal performance is good, with effluent stable below 15 mg/L. At this point, considering other pollutant removal, the wastewater treatment capacity can be appropriately increased.
As shown in Figure 3, as water temperature gradually decreased, the NH4+-N removal rate decreased from a maximum of 99.52% to a minimum of 74.77%, and effluent NH4+-N concentration increased from a minimum of 0.17 mg/L to 8.40 mg/L. Decreasing water temperature inhibits the activity of nitrifying and nitritifying bacteria, reducing NH4+-N removal. However, when water temperature >12°C, the average effluent NH4+-N concentration was 1.58 mg/L. When water temperature ≤12°C, the average effluent NH4+-N concentration increased to 6.58 mg/L, but effluent NH4+-N always met the Grade A standard of DB 34/3527-2019. When water temperature was 20~32°C, the average NH4+-N removal rate exceeded 96%. Combined with Figure 5 (b), the effluent NH4+-N concentration was below 2 mg/L in this range, indicating high nitrifying bacteria activity and excellent overall NH4+-N removal. When water temperature gradually decreased from 20°C to 12°C, the average NH4+-N removal rate still exceeded 90%, showing good removal, as research indicates water temperature >12°C is suitable for nitrifying bacteria growth, promoting nitrification. Therefore, NH4+-N maintained high removal rates in the 12~20°C range. When water temperature gradually decreased from 12°C to 0°C, the average NH4+-N removal rate still reached 80%. Existing research shows that nitrifying bacteria almost lose nitrification capacity at 0°C. However, the results of this study show that even at 0°C, the NH4+-N removal rate exceeded 75%, indicating good nitrification performance of this process at low temperatures. This is because the IFAS system in the A2O-MBBR section of this study has a long biofilm sludge age of up to about 1 month, making the nitrification rate in the biochemical tank far less affected by temperature than traditional activated sludge processes, significantly improving nitrification performance at low winter temperatures. Research by Wei Xiaohan et al. also indicates that the main reason for non-compliant NH4+-N effluent under low water temperature conditions is insufficient activated sludge age, with the impact of temperature on nitrifier activity being secondary. Therefore, although decreasing water temperature affected nitrifier activity to some extent, the sufficient sludge age in this process ensured NH4+-N removal at low temperatures. During the test period, the average effluent NH4+-N concentration was 3.50 mg/L, and the combined process exhibited good and stable nitrification performance.
2.2.3 Phosphorus Removal Performance
As shown in Figure 3, the TP removal rate varied little with water temperature changes, remaining stable above 94%. Combined with Figure 6, influent TP concentration ranged from 3.03~4.14 mg/L, and effluent TP concentration ranged from 0.14~0.28 mg/L, meeting the Grade A standard of DB 34/3527-2019. This process relies on the combined action of biological phosphorus removal (by PAOs) and chemical phosphorus removal (by PAC). When water temperature decreases, PAO activity is inhibited, affecting biological phosphorus removal. However, this process supplements with chemical phosphorus removal by dosing 3.7 g/d of PAC, maintaining a stable TP removal rate and reducing the impact of water temperature changes on phosphorus removal in the combined process. The A2O unit had the best TP removal performance. The anaerobic-anoxic unit effluent average TP concentration was 2.48 mg/L, with a removal rate of 32.61%. The aerobic unit effluent average TP concentration was 0.29 mg/L, with a removal rate of 59.51%. The overall TP removal rate for the A2O unit was 92.12%. The baffled design of the A2O-MBBR section can largely remove nitrate nitrogen carried in the recirculated mixed liquor, allowing anaerobic PAOs to release phosphorus more thoroughly in the anaerobic section and absorb phosphorus more fully in the aerobic section, enhancing biological phosphorus removal. Additionally, chemical phosphorus removal by dosing on one side of the aerobic MBBR tank maintained a stable TP removal rate, with effluent quality stably better than the Grade A standard of DB 34/3527-2019. Biological phosphorus removal in the A2O-MBBR section mainly occurs when PAOs in the baffled anaerobic tank use carbon sources to convert part of the organic matter and volatile fatty acids into polyhydroxyalkanoates (PHAs). When wastewater flows from the baffled anaerobic tank to the aerobic MBBR tank, PAOs then use PHAs as electron donors to complete phosphorus uptake. However, biological phosphorus removal performance is easily affected by PAO activity, and low water temperature limits PAO activity. Therefore, to achieve stable phosphorus removal, chemical phosphorus removal was incorporated in the process design. Additionally, the adsorption by the substrate layer in the carbon-based subsurface flow wetland and the growth of submerged plants in the ecological pond also absorb some phosphorus.
In summary, the setup operated stably during the test period, with good overall pollutant removal performance. The A2O-MBBR + CWs combined process achieved average removal rates of 68.40%, 89.45%, 73.94%, and 94.04% for TN, NH4+-N, COD, and TP, respectively. The average effluent concentrations were 11.69 mg/L, 3.50 mg/L, 26.9 mg/L, and 0.22 mg/L, respectively, all meeting the Grade A standard of DB 34/3527-2019. Research by Wu Qiong et al. indicates that A2O-MBBR is a composite process of activated sludge and biofilm, featuring large microbial quantity, long sludge age, high volumetric loading, small volume and footprint, strong resistance to shock loads, good effluent quality, and stable operation. Moreover, the denitrification performance of biofilm processes in winter is better than that of activated sludge processes, making it more suitable for treating low-temperature wastewater in winter. This is also the main reason for the good pollutant removal performance of the A2O-MBBR section in this study. The A2O-MBBR + CWs combined process in this study adds a CWs polishing treatment zone on the basis of the A2O-MBBR process, further enhancing the overall purification performance and operational stability of the process. The removal of TN and NH4+-N was less affected by seasonal water temperature changes, while the removal of COD and TP was almost unaffected by seasonal water temperature. During the test period, it exhibited strong resistance to shock loads, making it suitable for use in rural areas with large fluctuations in domestic wastewater quality and quantity.
2.3 Economic Analysis of the Combined Process
The costs of this combined process mainly include construction costs and wastewater treatment operation costs. Construction costs were for setting up the experimental setup, including purchasing tank bodies, ancillary electrical equipment, media, submerged plants, and pipe fittings, totaling approximately 3,000 CNY. Based on the maximum wastewater treatment capacity during the experiment of 0.18 m³/d, the construction cost per m³ of wastewater treated is approximately 16,700 CNY. Operation costs mainly arise from setup operation, including equipment energy consumption, chemical costs, sludge disposal costs, and labor costs. Electrical equipment includes: feed pump (power 2 W, Q=2.8 m³/d), recirculation pump (power 2 W, Q=2.8 m³/d), aerator (power 5 W, aeration rate=5 L/min), and peristaltic dosing pump (power 2 W). Calculated based on actual maximum usage power: feed pump 0.13 W, recirculation pump 0.19 W, aerator 1.25 W, dosing pump 2 W. Total actual usage power is 0.00357 kW, daily power consumption 0.086 kWh. Electricity consumption per m³ of wastewater treated is 0.48 kWh. Using industrial electricity price of 0.7 CNY/kWh, electricity cost is 0.33 CNY/m³. PAC chemical cost is about 2.4 CNY/kg, usage 3.7 g/d. PAC required per m³ of wastewater is 20.56 g, cost 0.05 CNY/m³. Sludge disposal cost = sludge quantity × unit volume sludge disposal cost. Dry sludge production per ton of water is 0.09 kg. Based on municipal WWTP sludge transportation and disposal unit price of 60 CNY/ton, sludge disposal cost per ton of water = 0.09 kg × 0.06 CNY/kg = 0.054 CNY. Since the pilot setup only required periodic inspection after operation, labor cost was estimated based on actual engineering experience. A 10,000-ton per day plant is operated by 1~2 persons. Assuming a single person's salary is 3,000 CNY/month, for 2 persons, the labor cost indicator is about 0.02 CNY/ton of water. Cost details are shown in Table 4. In summary, the operation treatment cost is approximately 0.46 CNY/m³. However, as wastewater treatment capacity increases, the construction and operation costs per ton of water would decrease. The construction and operation costs during the pilot test are for reference only.
3. Conclusions
The A2O-MBBR + CWs combined process showed good performance for rural domestic wastewater treatment. The removal of TP and COD was largely unaffected by water temperature changes. The average removal rates for TN, NH4+-N, TP, and COD reached 68.4%, 89.45%, 94.02%, and 73.94%, respectively. When water temperature ≤5°C, effluent quality stably met the Grade A standard of DB 34/3527-2019. When water temperature >5°C, effluent quality could meet the Grade A standard of GB 18918-2002 "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants". This process can efficiently utilize organic matter within the system as a carbon source to enhance denitrification, maintaining over 50% TN removal even at water temperatures as low as 0°C.
The optimal wastewater treatment capacity for the A2O-MBBR + CWs combined process in winter was 120 L/d, and 180 L/d in non-winter seasons. Seasonal water temperature changes (gradually decreasing from 32°C to 0°C) only had a certain impact on nitrogen removal by the combined process. The TN removal rate decreased from 79.19% to 51.38%, and the NH4+-N removal rate decreased from 99.52% to 74.77%. Even at 0°C, effluent quality stably met the Grade A standard of DB 34/3527-2019, and the NH4+-N removal rate still reached 74.77%. This benefits from the IFAS system, where a sludge age of up to 1 month ensured nitrification at low temperatures. The process operated stably during the test period, exhibiting strong resistance to water temperature changes.
The upfront A2O-MBBR process used two types of suspended biofilm carriers for microbial attachment, forming an IFAS system. The carbon-based subsurface flow wetland used multiple media fillers including sludge biochar, limestone, and zeolite, enhancing its filtration performance while providing ample attachment surface for microorganisms, improving its biological treatment capacity. The upfront A2O-MBBR process with IFAS has high biomass concentration. The rear CWs composite wetland serves as a polishing treatment stage, further treating the wastewater, making the overall system more resistant to shock loads.
The A2O-MBBR + CWs combined process is suitable for treating domestic wastewater in rural areas with large fluctuations in quality and quantity. It operates stably and efficiently, with a treatment cost of approximately 0.46 CNY/m³. Moreover, the A2O-MBBR+CWs process sections can be flexibly adjusted according to different effluent standards, scenarios, and purposes. This combined process can provide data reference and basis for rural domestic wastewater treatment projects in China, offer a resource utilization pathway for idle wasteland in rural areas, and has broad market application potential under the national trend of (highly emphasizing the improvement of rural environmental quality.